The present invention relates to a zoom lens, and in particular, to a zoom lens which is used for a digital still camera or for a video camera, and especially has a high resolving power suitable for a camera employing a high-density pixel type CCD, and has F number of about 2.8 at a wide-angle end and a ratio of variable magnification of about 2-3.
With the recent spread of personal computers and of handling of image data by the personal computers, digital still cameras for taking in image data have been demanded increasingly. Further, with a trend for densification of CCD, a demand for the zoom lens having higher image forming power has been increased. In addition, a compact one which is handy for carrying is demanded.
Zoom lenses with a magnification of about 3 which are suitable for a camera employing a solid-state image sensor such as CCD or the like have so far been disclosed in terms of technology, and examples thereof disclosed in TOKKAIHEI Nos. 10-39214, 11-52246 and 11-287953 and TOKKAI No. 2000-9997 are known.
However, in these conventional examples, there have been problems that distortion at a wide-angle end is extremely large, an angle of view at a wide-angle end is small and a total lens length is great.
The invention has been achieved in view of the problems stated above, and its object is to provide a zoom lens which has a ratio of variable magnification of about 2-3 as well as high image forming power and is suitable to be used for a digital still camera and a video camera both employing CCD of a high density pixel type.
The object mentioned above can be attained by either one of the following structures.
Structure (1)
A zoom lens having therein three lens groups including a first lens group having a negative refracting power, a second lens group having a positive refracting power and a third lens group having a positive refracting power in this order from the object side, in which the second lens group is composed of a positive lens, a cemented lens including a positive lens and a negative lens and one lens in this order from the object side, and when changing a magnification from the wide-angle side to the telephoto side, the first, second and third lens groups are moved on an optical axis so that a distance between the first lens group and the second lens group is reduced and a distance between the second lens group and the third lens group is increased for the magnification change, wherein the following conditional expression is satisfied;
0.4 less than fw/f2 less than 1.0xe2x80x83xe2x80x83(1)
wherein, f2 represents a focal length of the second lens group and fw represents a focal length of the total optical system at a wide-angle end.
Structure (2)
The zoom lens described in (1) wherein the first lens group has at least one aspheric surface and is composed of two pieces including a negative lens and a positive lens in this order from the object side, and the following conditional expression is satisfied;
xe2x80x83xe2x88x920.7 less than fw/f1 less than xe2x88x920.3xe2x80x83xe2x80x83(2)
wherein, f1 represents a focal length of the first lens group.
Structure (3)
The zoom lens described in (2) wherein the first lens group is composed of two pieces including a negative meniscus lens whose concave surface faces toward the image side and a positive meniscus lens whose convex surface faces toward the object side in this order from the object side.
Structure (4)
The zoom lens described in (1) wherein the first lens group is composed of three pieces including two negative lenses and one positive lens, and the following conditional expression is satisfied.
xe2x88x920.7 less than fw/f1 less than 0.3xe2x80x83xe2x80x83(2)
Structure (5)
The zoom lens described in (4) wherein the first lens group is composed of three pieces including a negative meniscus lens whose concave surface faces toward the image side, a negative lens and a positive lens in this order from the object side.
Structure (6)
The zoom lens described in (5) wherein the first lens group is composed of a negative meniscus lens whose concave surface faces toward the image side and a cemented lens including a negative lens and a positive meniscus lens whose convex surface faces toward the object side in this order from the object side.
Structure (7)
The zoom lens described in either one of (1)-(6) wherein the second lens group has an aspheric surface on at least one surface.
Structure (8)
The zoom lens described in (7) wherein at least one surface of the positive lens closest to the object in the second lens group has an aspheric surface.
Structure (9)
The zoom lens described in either one of (1)-(8) wherein the following conditional expression is satisfied;
|fw/f2i| less than 0.5xe2x80x83xe2x80x83(3)
wherein, f2i represents a focal length of the lens closest to the image in the second lens group.
Structure (10)
The zoom lens described in either one of (1)-(9) wherein the lens closest to the image in the second lens group is a meniscus lens whose convex surface faces toward the image side, and the following conditional expression is satisfied.
|fw/f2i| less than 0.4xe2x80x83xe2x80x83(4)
Structure (11)
The zoom lens described in either one of (1)-(10) wherein the lens closest to the image in the second lens group is a positive lens, and the following conditional expression is satisfied.
0.0 less than fw/f2i less than 0.4xe2x80x83xe2x80x83(5)
Structure (12)
The zoom lens described in either one of (1)-(10) wherein the lens closest to the image in the second lens group is a negative lens, and the following conditional expression is satisfied.
xe2x88x920.3 less than fw/f2i less than 0.0xe2x80x83xe2x80x83(6)
Structure (13)
The zoom lens described in either one of (1)-(12) wherein the following conditional expression is satisfied.
0.5 less than fw/f2 less than 0.7xe2x80x83xe2x80x83(7)
Structure (14)
The zoom lens described in either one of (1)-(13) wherein the third lens group is a single lens having a positive refracting power.
Structure (15)
The zoom lens described in (14) wherein the third lens group is a single lens having a positive refracting power of not less than 50 in terms of Abbe number.
Structure (16)
The zoom lens described in either one of (1)-(15) wherein the third lens group simply moves on an optical axis toward the object side when changing a magnification from the wide-angle side to the telephoto side, and satisfies the following conditional expression;
0.2 less than fw/f3 less than 0.7xe2x80x83xe2x80x83(8)
wherein, f3 represents a focal length of the third lens group.
Structure (17)
The zoom lens described in either one of (1)-(16) wherein the third lens group conducts focusing to cover from the object at the infinite point to the object at the short distance, and satisfies the following conditional expression.
0.2 less than fw/f3 less than 0.7xe2x80x83xe2x80x83(8)
In the zoom lens of the invention, a first lens group having a negative refracting power, a second lens group having a positive refracting power and a third lens group having a positive refracting power are provided, and when zooming from the wide-angle end to the telephoto end, each lens group is moved so that a distance between the first lens group and the second lens group is reduced and a distance between the second lens group and the third lens group is increased. When the second lens group is composed of a positive lens, a cemented lens including a positive lens and a negative lens and one lens in this order from the object side, a magnification can be changed effectively and the total optical system can be made to be compact.
To be more effective, it is preferable that the first lens group is structured by a negative meniscus lens whose concave surface faces toward the image side and by a positive meniscus lens whose convex surface faces toward the object side in this order from the object side, and has an aspheric surface, and this structure makes it possible to reduce the number of lenses and thereby to correct satisfactorily the distortion that is easily caused at the wide-angle side. Or, it is preferable that the first lens group is structured by a negative meniscus lens whose concave surface faces toward the image side, a negative lens and by a positive meniscus lens whose convex surface faces toward the object side in this order from the object side, and this structure makes it possible to correct satisfactorily spherical aberration caused mainly at the telephoto side.
It is preferable that the lens closest to the image side in the second lens group is a meniscus lens whose convex surface faces toward the image side, and this structure makes the second lens group itself to be compact, and it further corrects spherical aberration and coma satisfactorily by employing an aspheric surface.
The third lens group can correct the deviation of the focus point that is caused in the course of zooming, by moving on an optical axis. When focusing is necessary to comply with a change in a distance to an object, it is preferable to conduct the focusing by moving the third lens group. Though it is also possible to conduct focusing by moving the first lens group, a burden for the motor is greater in this case because the larger and heavier lens group must be moved, compared with an occasion to move the third lens group. In addition, when the first lens group is moved for the focusing, it is further made to be greater in size because a light flux in the course of advancing the first lens group needs to be secured. It is preferable that the third lens group is a single lens having Abbe number of 50 or more. With this structure, chromatic aberration generated in the third lens group can be lightened.
Next, each conditional expression will be explained. Conditional expressions (1) and (7) are those to regulate a refracting power of the second lens group and to obtain an excellent image forming power with a fully compact lens. When the value is above the lower limit of the conditional expression (1), the total optical system keeps proper, and miniaturization thereof can be realized. Further, when the value is below the upper limit, various aberrations generated in the second lens group becomes small, and spherical aberration and coma, in particular, can becomes small and excellent correction can be done, even when an aspheric surface is used in the second lens group. It is preferable to be within a range of conditional expression (7) to obtain a lens which is compact and is powerful.
Conditional expression (2) is one to regulate a refracting power of the first lens group and to obtain an excellent image forming power with a fully compact lens. When the value is above the lower limit of the conditional expression, various aberrations generated in the first lens group becomes small, and in particular, excellent correction for distortion at a wide-angle end, transverse chromatic aberration and for spherical aberration at a telephoto end can be achieved. Further, when the value is below the upper limit of the conditional expression, miniaturization of the total optical system can be realized.
Conditional expression (3), conditional expression (4), conditional expression (5) or conditional expression (6) is one to regulate a refracting power of the lens closest to the image side in the second lens group. When the value satisfies the conditional expression of conditional expression (3), various aberrations generated in the second lens group can be made smaller, and spherical aberration and coma, in particular, becomes smaller, and miniaturization of the total optical system can be realized. For obtaining a lens which is more compact and more powerful, it is preferable to be within a range of conditional expression (4), conditional expression (5) or conditional expression (6).
Conditional expression (8) is one to regulate a refracting power of the third lens group and to obtain an excellent image forming power with a fully compact lens. When the value is above the lower limit of the conditional expression, a back focus can be shortened, an amount of movement of the third lens group for focusing is increased and the total length can be decreased, which is preferable. Further, when the value is below the upper limit of the conditional expression, coma and a curvature of the field generated in the third lens group can be made small, and correction can be conducted very well, even when an aspheric surface is used in the third lens group.